Merge add_startup_object cmake functions

[michaelrj-google]

Currently each OS target defines its own add_startup_object function (e.g. linux:

llvm-project/libc/startup/linux/CMakeLists.txt

Line 53 in 51bceb4

function(add_startup_object name)

)
We should merge these into one function and put it alongside add_entrypoint_object etc. in /cmake/modules.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Michael Jones (michaelrj-google)

Currently each OS target defines its own `add_startup_object` function (e.g. linux: https://github.com/llvm/llvm-project/blob/51bceb46f8eeb7c3d060387be315ca41855933c2/libc/startup/linux/CMakeLists.txt#L53) We should merge these into one function and put it alongside `add_entrypoint_object` etc. in `/cmake/modules`.